The German Patent Application DE 43 32 735 A1 “Verfahren zur digitalen Erzeugung eines komplexen Basisbandsignals” describes an algorithm for the generation of a complex baseband signal which makes use of a polyphase-filter. This Patent Application also describes that the branch filters of the polyphase-filter should be realized as allpass filters.
The paper “Entwurf und Realisierung diskreter Filterbänke”, published by Shaker, ISBN 3-8265-0366-X, from Torsten Leickel explains how to design such polyphase-filters with allpass branch filters.
The design of polyphase filters with allpass branch filters is well known in the art.
In prior systems, the input signal of the polyphase-filter is multiplexed into N different ailpass filters. Therefore, N ailpass filters have to be realized with N being the decimation factor of the polyphase filter. This design leads to high realization costs for the polyphase-filter. Also, only a restricted amount of IF-frequencies can be realized with this structure, since the IF-frequencies can only be chosen to FIF=M·F+L·F/N with F being the sampling rate of the filter input signal, L being a natural constant between
                    -        N            -      1        2    ⁢          ⁢  …  ⁢          ⁢            N      -      1        2  and m being a natural constant.
Further, EP 0 597 255 A1 “Empfänger für ein digitales Rundfunksignal mit digitaler Signalverarbeitung” discloses an efficient realization of an algorithm for the generation of a complex baseband signal. To optimize the realization of the digital signal processing a switchable allpass filter is used. However, this realization has the disadvantage that no digital neighbour channel suppression/noise shaping can be realized and the IF-frequency can only be chosen to FIF=m·F±F/4 with F being the IQ-filter input sampling rate and m being a natural constant.
The disadvantage of using switchable ailpass filters is that no digital channel suppression/noise shaping can be realized. Furthermore, the IF frequency can only be chosen to be FIF=m·F±(F/4) with F being the IQ filter input sampling rate and m being a natural constant.
However, prior IQ-generators are quite restricted in respect to the used Intermediate Frequency (IF) and in respect to possible sampling frequencies of the A/D converter converting the IF signal into a signal suitable for digital baseband processing since the output frequency of the generation is fixed according to certain standards and the input frequency of the IQ generation (i.e., the IF frequency) strongly depends on the used IQ-filter and the needed output frequency.